Stents are commonly used medical implants designed to prop open the inside of a patient's vessels or body lumens. In most cases, stents are used to treat occlusions that have formed within a body lumen such as a coronary artery or the biliary duct. The typical stent is a metal cylindrical tube that has been rendered flexible by its specific construction. A stent may be formed of a metal tube that has been etched or cut out to weaken the walls of the tube to such a point that the tube provides support to the body lumen in which it is placed, but offers enough flexibility so as to be deliverable on a catheter to the site in the body where it is deployed. As well, stents have been made by weaving or knitting metal wires to create a tube having good flexibility, sufficient radial strength and deliverability. Other stents have been made of polymers, biological materials, fabrics and combinations thereof.
In all cases, the stent must be designed such that it is deployable by catheter or similar stent delivery system, as it is desirable for stent placement procedures to be minimally invasive. By virtue of the openings in the wall of the stent, a stent may be reduced in radius when positioned on the delivery catheter. The stent may then be delivered through a small percutaneous opening in the body of the patient or through a natural orifice such as the mouth. In any event, low profile and controllability are necessities for delivering stents to body lumens. Once in position, a stent may be deployed by removing a constraint, such as a sheath, which has maintained low profile of the stent during delivery. Once the sheath is removed, the stent will self expand to support the vessel. Another common deployment method is to inflate a balloon inside the stent in order to force the stent open. The balloon is usually part of the delivery catheter, but it may be administered following an initial stent deployment by self-expansion or another balloon.
When deployed, stents will take their designed and manufactured shape which is, as stated above, a cylindrical tube. Stents can conform somewhat to the anatomy of the lumen in which it is deployed. As well, various designs and materials have been explored that allow the deployed stent to bend in place with the anatomy. Bending may occur in three dimensions. As a result of the straight cylindrical geometry of current stents, a patient's anatomy is induced to conform to the deployed stent shape rather than the stent conforming to the anatomical shape. A flexible material or design may relieve the influence of the stent on the vessel morphology, yet these factors cannot ideally suit a patient's anatomy.
The influence of a straight stent in a naturally curved lumen may reduce the advantages of relieving the occlusion in the lumen. In areas of fluid flow, as in the arterial tree, a transition from the artery's natural curvature to a straighter section supported by a stent may induce localized turbulence in the blood. Turbulence may itself lead to thrombosis or emboli formation, which can, in turn, occlude the vessel or vessels downstream. Additionally, a straight stent in a curved vessel will naturally exert higher stresses at the ends of the stent upon the lumen wall. Localized stresses may incite an injury response and potentially areas of increased restenosis. As well, the exertion of local stress on weakened vessels, as is often the case for patient's suffering from various diseases, may lead to complications such as erosion, abrasion or other types of injury on the sensitive cellular linings. Exposure of inner layers of a vessel in the vascular system will incite localized thrombus formation and possible emboli. Such injuries may also lead to vessel wall hyperplasia and possibly restenosis of the stented vessel. These factors are complicated by lumens that are submitted to pressure cycles, as in the arterial tree.
In addition to the influences of arterial pressure, surrounding tissues may have a marked influence on the intended effects of the stent. A stent in a coronary vessel is subjected to constant contractions and expansions of the heart. If the stent were imparting a straightening force upon the coronary artery, this force would be amplified at various points in the cardiac cycle. Another deleterious effect of surrounding tissue may be eventual stent migration. A stent that does not conform to the surrounding vessel may be induced to move along the axis of the vessel to an area of less longitudinal stress, especially during moments of surrounding tissue contraction. As might be expected, cardiac tissue is not the only type of contractile tissue. A stent located in a leg artery, vein or duct may be subjected to the surrounding forces of the leg muscles. In such a case, the effect of the patient's weight over an implanted stent may further exacerbate the stress between the vessel and the stent. Finally, stented lumens that are subjected to straightening from a tortuous natural state will suffer from a magnified effect of the stresses imposed on the lumen in both the axial and radial directions.
Further examples of body lumens that would benefit from an anatomically shaped stent include but are not limited to arteries, veins, cerebral vessels and lumens, biliary and pancreatic ducts, lymph ducts, the gastrointestinal or GI tract, the aorta, nasal passages, ear canals, tear ducts, the ureters, the intestines, reproductive organs and pulmonary tracts.
Other anatomical features that may be necessary to accommodate include vessel branching, tapering, flaring, non-round cross-sections and multiple curvatures. Each of these factors represents a non-straight tube anatomy that may be negatively affected by treatment with a simple straight tube stent.
As exemplified above, there exists a need to provide for stents that can be deployed by minimally invasive procedures wherein such stents are shaped to assume the geometry of the body lumen in which they are placed and further wherein the stents successfully support the lumen without the detrimental effects caused by imposing an unnatural shape upon a body lumen.